Electric motors receive electrical energy and convert the electric energy into mechanical energy or torque. The rotary direct current electric motor delivers mechanical power by means of a rotating shaft extending from one or both ends of its enclosure. When a rotary direct current motor is first connected to an electric supply, there is a large surge of current because nothing in the motor opposes the flow of current except the armature resistance, which is very low. As the motor accelerates the counter emf increases which causes the armature current to decrease. The current decreases as the motor accelerates until a point of equilibrium is reached where the current drawn by the motor from is the power source just balances the requirement of the driven load at a constant speed.
As long as the motor-driven load remains constant, the motor speed remains constant. If the load on the motor is increased, the motor speed decreases. This decrease in speed causes a decrease in the counter emf, which in turn allows the motor to draw more current from the power supply, which in turn causes the motor to develop more torque to carry the increased load at a slightly lower speed. If the motor load continues to increase, the motor will continue to develop additional torque and continue to slow down. As the load continues to increase the motor will have to be turned off to prevent damaging the motor. The prior art utilized electrical circuit breakers like fuses to protect electrical devices from excessive currents. In a typical fuse, there is a time delay between the first sensing of the high current and the blowing of the fuse. The blowing of the fuse caused the motor to stop. Thus, a problem with the prior art was that intervention would be required for the fuse to blow and time would be required to replace the fuse.
Prior art Motor systems employed in paper handling equipment used a Positive Temperature Coefficient resistor (PTC) or a Polyfuse to control excessive current. The positive temperature coefficient resistor was placed in series with the motor and acted somewhat like a fuse. If the Motor stalled, the current drastically increased and the PTC heated up. When the PTC heated up, the resistance increased, which caused the current to decrease, thus preventing the motor from becoming a fire hazard. The use of PTC type devices also eliminates the need to replace fuses. Since the motor can be shut off, the PTC device cools and the motor can be returned to normal service.
If a paper jam would occur in paper handling equipment that utilized PTC type devices and fuses, typically it would take too long for the PTC device to heat up and limit the current before the fuse would blow. The foregoing was true even if a slow blowing fuse with a time delay was used. Thus, a simple paper jam may necessitate a service call to repair the damage.
Many different types of systems utilize electric motors to move material through the system, i.e., photocopiers, integrated mailing systems, etc. The above systems utilize closed-loop motion control circuits to monitor and control the torque of individual motors. The torque limiting was accomplished by utilizing analog set points and analog circuitry to measure the current required by the motor. If the current exceeded a preset threshold, the torque limiter would reduce the current or turn off the motor.
A disadvantage of the foregoing is that the system was inflexible, since the system was dealing with fixed set points. The above system would turn off the motor when the set point was reached, regardless of the existing machine or motor operation. The system may turn off the motor when the motor should not be turned off, i.e., the set point was set too low. The above system was also subject to false triggers from transient currents. Furthermore, if the current thresholds were set too high, the drivers could fail and the fuse protecting the circuit could blow, necessitating a service call to repair the motor or machine.